Although the ZIP code has many advantages as a geographic unit to use when mapping health data, such as its widespread accessibility as a geocode in health databases and its small spatial size, it also has a number of limitations, such as the fact that true boundaries of ZIP codes are not known and change through time. It is well known that the choice of geographic unit can affect the interpretation of maps and the results of spatial analyses, a phenomenon known as the modifiable areal unit problem (MAUP).1 As such, it is important to consider the advantages and disadvantages of the ZIP code, as well as other geographical units, before deciding on a unit that will yield the most accurate maps and analysis of cancer data. In this chapter, we examine the advantages and disadvantages of the ZIP code, explore examples of research that have used the ZIP code, and address key questions about ZIP code use.